server-auth-key

server-auth-key is a variable defined in `server.el'.
Its value is

nil

Documentation:
Server authentication key.
This is only used if `server-use-tcp' is non-nil.

Normally, the authentication key is randomly generated when the
server starts. It is recommended to leave it that way. Using a
long-lived shared key will decrease security (especially since
the key is transmitted as plain-text).

In some situations however, it can be difficult to share randomly
generated passwords with remote hosts (e.g., no shared directory),
so you can set the key with this variable and then copy the
server file to the remote host (with possible changes to IP
address and/or port if that applies).

Note that the usual security risks of using the server over
remote TCP, arising from the fact that client-server
communications are unencrypted, still apply.

The key must consist of 64 ASCII printable characters except for
space (this means characters from ! to ~; or from code 33 to
126). You can use M-x server-generate-key to get a random key.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.